The kidneys filter waste products and excess fluid from the bloodstream to produce urine. Monitoring this filtration capacity is fundamental to diagnosing and managing kidney disease. Healthcare providers assess this function using the estimated Glomerular Filtration Rate (eGFR) as the standard index. Modern medical practice frequently integrates Cystatin C, a newer blood marker, alongside the eGFR calculation to improve accuracy. This combined approach offers a more precise understanding of kidney health, especially when traditional measures may be unreliable.
Cystatin C: The Filtration Marker
Cystatin C is a small protein produced at a constant rate by virtually all nucleated cells throughout the body. Its stable production, which is largely unaffected by factors like muscle mass, diet, or sex, makes it an ideal marker for kidney function. This low-molecular-weight protein is freely filtered across the glomeruli, the kidney’s primary filtering units. The amount of Cystatin C in the blood is inversely related to the kidney’s filtration efficiency; as filtration slows, the blood concentration rises.
After filtration, Cystatin C is almost entirely reabsorbed and metabolized by the cells lining the renal tubules, rather than being excreted into the urine. This process ensures that the protein’s blood concentration is determined exclusively by the glomerular filtration rate. Its independence from non-kidney factors provides a reliable reflection of the kidney’s performance.
What Estimated Glomerular Filtration Rate Measures
The Glomerular Filtration Rate (GFR) represents the volume of fluid filtered by the kidney glomeruli per minute, serving as the standard measure of kidney function. A high GFR indicates efficient filtering, while a low GFR suggests kidney impairment. Directly measuring the GFR is a complex, time-consuming, and expensive procedure.
Because direct measurement is impractical, clinicians use an Estimated Glomerular Filtration Rate (eGFR), which is a calculated score derived from a simple blood test. The eGFR calculation uses the concentration of a filtration marker, such as Cystatin C, along with patient demographic data, including age and sex. The eGFR value is standardized to a body surface area of 1.73 square meters, which allows for comparisons across different individuals. This formula provides a quick approximation of the actual GFR, allowing for routine monitoring of kidney health.
The Clinical Advantage of Combining Cystatin C and eGFR
The traditional method for calculating eGFR relied solely on serum creatinine, a waste product of muscle metabolism. Creatinine levels are significantly influenced by muscle mass, diet, and certain medications, often leading to inaccurate eGFR results. For example, individuals with low muscle mass, such as the elderly, may have deceptively low creatinine levels, falsely suggesting a healthier eGFR.
Cystatin C overcomes these limitations because its blood concentration is independent of muscle mass or protein intake, providing a more accurate reflection of the filtration rate. This makes Cystatin C-based eGFR valuable when creatinine estimates are unreliable. Cystatin C can also detect early-stage kidney disease more sensitively than creatinine, sometimes identifying dysfunction even when creatinine levels are normal.
The most robust assessment of kidney function combines both markers into a single calculation, known as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine-cystatin C equation. This dual-marker eGFR provides a more precise and reliable estimate across a wider range of patients and disease stages. Guidelines often recommend using Cystatin C as a confirmatory test when the creatinine-based eGFR falls into a borderline range (e.g., 45 to 59 mL/min per 1.73 m²), ensuring accurate diagnosis and staging.
Interpreting Your Kidney Function Results
The final eGFR number is reported in \(\text{mL/min/1.73 m}^2\) and is used to classify the stage of kidney function. A result of 90 or higher is considered normal. An eGFR between 60 and 89 may suggest mild kidney damage, especially if other signs of damage, like protein in the urine, are present.
A result below 60 confirms the presence of Chronic Kidney Disease (CKD) if the reduced function persists for three months or more. The CKD stages are categorized based on this number:
- Stage 3 (eGFR 30–59): Moderate reduction, often split into 3a (45–59) and 3b (30–44).
- Stage 4 (eGFR 15–29): Severe reduction.
- Stage 5 (eGFR below 15): Kidney failure.
Patients should always discuss their eGFR results and the associated CKD stage with a healthcare provider. The provider will consider these numbers alongside other clinical information, such as urine tests and medical history.